It has long been recognized that lead-acid storage batteries tend to emit hydrogen and oxygen while the battery is being charged and that the gases so emitted must be vented from the storage battery casing. Since storage batteries are used in environments where sparks occur, the usual automobile being a typical example, it is well known that venting of the hydrogen and oxygen from the battery causes a serious hazard of explosion. Prior-art workers have sought to provide special caps for the filling openings of the battery cells, for example, with the caps so designed as to vent the explosive gases to the atmosphere in a manner intended to minimize chances that an explosion would occur and also minimize chances that should an external explosion occur, the explosion could propagate through the vent cap into the interior of the cap and the battery. Many such vent caps have been proposed, typically including tortuous vent paths, relatively small vent apertures, and porous vent bodies such as membranes, ceramic plugs or sintered metal plugs. In recent times, the use of porous vent plugs has received considerable attention and such plugs have been effective to prevent flame from propagating back through the plug into the battery. Unfortunately, it has been found that such devices have the disadvantage of establishing persistent Bunsen burner-like flame, external to the plug, and that persistence of the flame generates such a high temperature as to e.g. melt the surrounding polymeric material in which the plug is mounted, so that the plug is blown from its mounting and explosion of the battery results.
Though flame behavior has not been explained to complete satisfaction in terms of chemical kinetics and transport properties of gas molecules, it is accepted that the flame established by burning a combustible gas mixture of given temperature, pressure and composition will not pass through an orifice smaller than a certain minimum size, commonly referred to as the "flame quenching distance". Research on this subject is reported in The Quenching of Laminar Oxyhydrogen Flames By Solid Surfaces, Raymond Friedman, Third Symposium on Combustion, Flame and Explosion Phenomena, Williams and Wilkens Company, Baltimore, Maryland, 1949, and Survey of Hydrogen Combustion Properties, Isadore L. Drell et al., 1958, NACA-TR-1383.
The flame quenching distance for a stoichiometric mixture of hydrogen and oxygen has been established as 0.0075 in. (0.019 cm.) under specific test conditions, and varying the relative proportions of hydrogen and oxygen does not appear to change the flame quenching distance dramatically. The flame quenching distance appears to be relatively independent of the material of which the elements defining the orifice are made.
Success of prior-art anti-explosion vent caps of the type employing porous ceramic plugs and the like can be explained by the fact that the capillary passages presented by the porous plug are of a transverse dimension smaller than the flame quenching distance for a mixture of hydrogen and oxygen, so that a flame cannot be propagated back through the plug into the battery. Failure of such devices, as a result of the heat caused by a persistent external flame, appears to be caused by the fact that, since the capillaries are immediately adjacent to each other and concentrated in a relatively small area, the hydrogen-oxygen mixture is emitted from the capillaries in a mode which is favorable to both ready ignition and persistent burning.